The invention disclosed and claimed herein pertains generally to the field of apparatus which is used to isolate a DC voltage source from a voltage measurement or monitoring system. More particularly, the invention pertains to the above apparatus wherein source voltage may be very high, and the monitoring system is to be selectively coupled to the source by means of an electronic switch. Even more particularly, the invention pertains to apparatus of the above type wherein optical-isolator devices are employed to provide high voltage barriers between a voltage measurement system and an electronic switch which is selectively coupled to the cells of a high voltage battery.
It is well known in the art that if one of the cells included in a multi-cell silver-zinc battery develops an internal short, a great deal of heat can be generated in the cell while the battery is charging or discharging. Such heat may severely damage the cell, the battery or surrounding equipment. Generally, in order to detect a shorted cell before a "hot cell" condition develops, it is necessary to periodically measure the individual voltages of every cell in the battery, and to compare such measured voltages to determine if one cell voltage is significantly less than the others. If a silver-zinc battery is very large, comprising in excess of 100 series-connected cells, the task of periodically testing individual cell voltages may be quite tedious.
In U.S. Pat. No. 3,786,343, issued to Ehlers on Jan. 15, 1974 and assigned to the U.S. Navy, the "hot cell" problem is discussed in some detail, and a system is disclosed which sequentially switches each of the cells of a silver-zinc battery to a voltage monitoring system. While the system of Ehlers has reduced some of the tedium in detecting shorted battery cells, it has also been found to have certain disadvantages. In Ehlers, mechanical relays are employed to connect respective battery cells to the monitoring system. Such relays may have oxidation on their contacts, causing errors in voltage readings. Also, each mechanical relay performs millions of connect and disconnect operations over a comparatively short period of time, whereby frequent relay replacement is required. It is also possible for a mechanical relay to stick, and to therefore remain closed, at the conclusion of a cell measurement operation. As a result, two cells can possibly become connected to the monitoring system at the same time, a situation which could be very destructive.
If a solid state electronic switching system could be used to replace the above system of mechanical relays, the various disadvantages arising from use of relays would be eliminated. However, a silver-zinc battery which includes a large number of series connected cells may have a total voltage, when fully charged, on the order of 750 volts. When the contacts of a mechanical relay are physically separated, the relay can generally withstand such high voltages. However, available electronic switches, even when in an open mode, cannot withstand voltages exceeding a limit which is on the order of 25 volts. It is therefore very important that an electronic switch connected to a high voltage battery is maintained in electrical isolation from a battery voltage monitoring system. Otherwise, high battery voltage may force a DC path through the switch to the monitoring system, severely damaging both.
In the present invention, apparatus is disclosed which successfully employs an electronic, rather than a mechanical relay, switching system to enable periodic measurement of the voltage of each of the cells of a high voltage battery. Optical means are employed to enable the switching system to receive control signals from a voltage monitoring system, and to couple voltage data thereto, while electrical isolation is maintained therebetween.